<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style type="text/css">
	.parent{
		background-color: yellow;
		border: 2px red solid;
		padding: 15px;
	}
	.a1{
		background-color: green;
		border: 2px green solid;
		padding: 10px;
		/* 设置盒子1是绝对定位absolute */
		position: absolute;
		/* 将盒子1向左(相对于浏览器的边框)靠齐，与浏览器的边框距离为0; */
		left: 0px;
		/* 将盒子1向上(相对于浏览器的边框)靠齐，与浏览器的边框距离为0*/
		top: 0px;
	}
</style>
</head>
<body>
<div class="parent">
	<div class="a1">盒子1</div>
	</div>
</body>
</html>